There are three basic types of projectile propulsion systems: the closed breach gun, rocket motors, and recoilless rifles. A fourth type, the Davis Gun, has never been weaponized for fire from enclosure capability.
The closed breach gun system works by the combustion of a propellant charge within a combustion chamber. As the propellant charge burns, pressure increases and accelerates the projectile down a barrel (gun tube). The closed breach gun system has substantial recoil, visual signature, and acoustic signature. In this type of system, the combustion gases exit the barrel at the muzzle and only the projectile travels down range.
The majority of current shoulder-launched munitions utilize rocket motor propulsion systems. Rocket motors function by combusting propellant within a pressure vessel, which is typically attached to the warhead. The high-pressure gases generated by the propellant combustion are expelled through a throat and out a nozzle, generating thrust or forward motion of the projectile within the launcher. This type of propulsion system can be used to minimize recoil felt by the user; however, it also has substantial visual signature and acoustic signature.
Recoilless rifles are a hybrid of the closed breach gun system and the rocket motor. This type of propulsion system functions by combusting propellant within the combustion chamber and the exhaust gases travel through a throat and a nozzle. However, both the throat and the nozzle are permanently installed into the launch tube and do not travel down range with the projectile. The nozzle within the launch tube is used to balance recoil. This type of propulsion system also has substantial visual signature and acoustic signature.
U.S. Pat. No. 6,971,299 utilizes a high-pressure and low-pressure system with a countermass. The system reports utilizing constrictive gas outlets within the high-pressure section prior to the low-pressure section. However, this reference does not report any technical features to ensure that the internal ballistic pressure reaches a specific limit prior to release of the projectile and interaction of the combustion gases with the countermass.
U.S. Pat. No. 8,448,556 also utilizes a high-pressure and low-pressure system with a countermass. The countermass reportedly maintains a proportionately high gas pressure in the countermass chamber, to balance the backward recoil from the projectile. The system reports improved control of gas flows from the high-pressure part to the low-pressure part regulated by the projectile in the launch tube and minimal recoil forces due to arrangement with an internal expansion nozzle coupled to the low-pressure part. The projectile is positioned in a first start position in the launch tube, where the projectile blocks the gas-openings, and where the projectile, upon ignition of the propellant charge, moves forward in the launch tube to further positions, where the gas-openings, successively, are unblocked by the projectile.
U.S. Pat. No. 8,220,376 utilizes a system in which the forward direction recoil disengages the launching tube from the firing and supporting unit. Thus, the countermass is designed such that a forward-directed recoil is obtained, and the launching tube is designed to be disengageable from the firing and supporting unit during the forward-directed recoil. This system purportedly balances recoil by transferring kinetic energy generated during firing to the launching tube.
U.S. Pat. No. 7,353,739 utilizes a separate countermass container, which is placed within the barrel. This reference reports that air is purposely trapped within the countermass container as a means to compensate for thermal expansion and contraction effects within the countermass. This countermass utilizes viscosity-changing additives, as well as the addition of micro-balloons as a means to bind and retain the countermass fluid.